1. Field of the Invention
The present invention relates to a lens-fitted photo film unit, and more particularly to a lens-fitted photo film unit having a stop changing device.
2. Description of the Related Art
Various types of lens-fitted photo film units are manufactured and are on the market. The lens-fitted photo film unit has a simple photographing mechanism and is loaded with a photographic film at the time of manufacturing. In the lens-fitted photo film unit, a stop and a flash-light amount are fixed for the purpose of simplifying its structure and reducing its cost. Due to this, it is caused to narrow a range of a photographic condition for properly taking a main subject and the background thereof. When subject brightness and main-subject distance are out of the range of the photographic conditions, photographic image quality deteriorates.
In order to solve the above problem, it is considered to incorporate a stop changing device into the lens-fitted photo film unit. This stop changing device switches a larger stop and a smaller stop in two steps to simply perform exposure adjustment. Such a stop changing device comprises a stop plate formed with a smaller-stop aperture. When the subject brightness is high, the stop plate is positioned at a photographic optical path to set the smaller stop. When the subject brightness is low, the stop plate is evacuated from the photographic optical path to set the larger stop.
As to mechanisms for moving the stop plate of the stop changing device, there are some types of system, for example, a subject-brightness measuring system and a flash interlocking system. In the subject-brightness measuring system, a solenoid is driven in accordance with a photometry value to move the stop plate. In the flash interlocking system, the stop plate is moved in association with an operation for sliding a flash switch. The stop changing device of the subject-brightness measuring system drives the solenoid so as to move the stop plate in accordance with a detection value of the subject brightness obtained from a light receiving element. The smaller stop is set in the case of high subject brightness and the larger stop is set in the case of low subject brightness. Meanwhile, in the stop changing device of the flash interlocking system, the stop plate is positioned so as to be associated with the flash switch. When the flash switch is turned off to take a picture without the flash light, the smaller stop is set. When the flash switch is turned on to take a picture with the flash light, the larger stop is set.
In the meantime, the lens-fitted photo film unit has a extremely simple structure regarding a taking lens, on account of reducing the cost. In general, the taking lens is constituted of one or two lenses. Recently, the taking lens having a structure of two groups and two lenses is used in view of an advantage in correcting an aberration. The taking lens of such a structure is held by a lens holder, which has a cylindrical shape and is disposed in front of a fixed aperture-stop. When this kind of the taking lens is combined with the foregoing stop changing device, the stop plate formed with the smaller stop aperture is inserted between the two lenses to position the smaller stop aperture at a front side of the aperture-stop.
It is required for the lens-fitted photo film unit to further reduce the cost. In view of this, it is considered to combine the stop changing device not only with the taking lens of the above structure but also with the taking lens of a single-lens structure. By decreasing a number of parts, the cost may be further lowered.
However, it is difficult to correct the aberration when the taking lens has the single-lens structure. As a stop diameter becomes large, the aberration further increases. Therefore, the image quality is likely to deteriorate at the time of the larger stop. In the above-described stop changing device, the larger stop is set when the subject brightness is low. If this stop changing device is combined with the taking lens of the single-lens structure, the image quality is likely to further deteriorate at the time of the larger stop. Because, lowness of the lens performance concurs with the low subject brightness.
In view of the foregoing, it is a primary object of the present invention to provide a lens-fitted photo film unit in which a stop changing device is combined with a taking lens having a single-lens structure.
It is a second object of the present invention to provide a lens-fitted photo film unit which is simplified whereas a stop changing device is combined with a taking lens having a single-lens structure.
It is a third object of the present invention to provide a lens-fitted photo film unit in which its cost is reduced whereas a stop changing device is combined with a taking lens having a single-lens structure.
In order to achieve the above and other objects, the lens-fitted photo film unit according to the present invention comprises a taking lens of a single lens, a lens base for holding the taking lens, and a stop changing device for moving a stop plate disposed behind the taking lens. The stop plate is formed with a stop aperture and is movable perpendicularly to a photographic optical axis.
The stop changing device moves the stop plate to change a stop of the lens-fitted photo film unit. The stop plate is moved between a smaller-stop position where the stop aperture of the stop plate is positioned at a photographic optical axis, and a larger-stop position where the stop aperture is evacuated from the photographic optical axis.
The lens base is provided with a concave into which the stop plate is inserted. The concave is formed inside a lens-holding rib integrally formed with the lens base. When the stop aperture of the stop plate is set to the smaller-stop position, the stop plate enters the concave. When the stop aperture is moved from the smaller-stop position to the larger-stop position, the stop plate is evacuated from the concave.
According to the present invention, by using the photographic film of high sensitivity, it is possible to properly perform the exposure at a low-brightness range. Moreover, in the case of high brightness, the stop is changed to the smaller stop so that high image-quality may be obtained. Although the taking lens of the single-lens structure is used, it is possible to obtain image quality corresponding to a taking lens of two-lens structure. A number of parts may be reduced and the construction is simplified to lower the cost.